1. Field of the Invention
The present invention relates to a training detection apparatus for detecting a training sequence sent from a transmission-side modem, for example, to a training detection apparatus capable of detecting training signals based on Segment 3/1 of the CCITT recommendation V.27ter/bis and Segment 2/1 of V.29/V.33, or Segment 4/2 of the CCITT recommendation V.27ter/bis and Segment 3/2 of V.29/V.33.
2. Prior Art
In a CCITT recommendation V.27ter or V.29 type modem, after transmission- and reception-side modems are connected through a communication line, the transmission-side modem outputs a predetermined training signal sequence (turn-ON sequence) so as to initialize signal processors of the reception-side modem.
For example, the training signal sequence has a format shown in Table 1 in a CCITT recommendation V.27ter type modem.
In a CCITT recommendation V.29 type modem, the training signal sequence has a format shown in Table 2.
In the CCITT recommendation V.27ter type modem shown in Table 1, the format of Segment 3 corresponds to a continuous 180.degree. phase inverted pattern on a line during a) a 14-symbol interval or b) a 50-symbol interval.
Segment 4 corresponds to an equalizer adjustment pattern, and is formed by the third bits obtained by dividing a pseudo random sequence generated by the following polynomial by 3 bits. EQU 1+X.sup.-6 +X.sup.-7
In the CCITT recommendation V.29 type modem shown in Table 2, Segment 1 corresponds to a preparation period for Segment 2 and the subsequent segments.
Segment 2 is an element, used in AGC adjustment, reception carrier timing extraction/synchronization, and the like, for alternately transmitting two signal elements. More specifically, a signal element (A) transmitted first has a relative amplitude value "3" and an absolute phase of 180.degree.. An element (B) transmitted secondly depends on the data speed. Signal elements in data signals will be described in detail later.
Segment 3 is an element for initializing an equalizer, and transmits two signal elements according to an equalizer adjustment pattern. A first signal element (C) has a relative amplitude value "3" and an absolute phase of 0.degree.. A second signal element (D) depends on the data speed. The equalizer adjustment pattern is generated according to a pseudo random sequence generated by the following polynomial. EQU 1+X.sup.-6 +X.sup.-7
Segment 4 is an element for attaining synchronization of a scrambler and a descrambler, and is generated according to a pseudo random sequence generated by the following polynomial. EQU 1+X.sup.-18 +X.sup.-23
The reception-side modem receives the training signal sequence, and initializes an AGC, an automatic equalizer, and the like as main constituting blocks of a receiving section.
In Tables 1 and 2, Segment 4 of the V.27ter type modem and Segment 3 of the V.29 type modem are segments for adjusting tap coefficients in an initialization stage so as to realize inverse characteristics of a communication line. The initialization of the reception-side modem is started from the beginning of Segment 3 of the V.27ter type modem and Segment 2 of the V.29 type modem, which are received earlier than the equalizer adjustment segments.
FIG. 6A shows the format of a demodulated baseband signal in Segment 3 at an 8-phase phase modulation rate of 4,800 bps (1,600 baud) of the V.27ter modem.
Continuous 180.degree. phase inverted signals indicated by black dots . are received. FIG. 6B shows frequency components of the demodulated baseband signal in this segment.
As shown in FIG. 6B, the demodulated baseband signal at this time has a frequency 1/2 a baud rate f.sub.b.
More specifically, the demodulated baseband signal has line spectrums at .+-.f.sub.b /2=.+-.1,600/2=.+-.800 Hz.
FIG. 7A shows the format of a demodulated baseband signal in Segment 2 at a 16-value orthogonal amplitude modulation rate of 9,600 bps (2,400 baud) of the V.29 modem.
In FIG. 7A, points surrounded by .largecircle. indicated by (A), (B), (C), and (D) correspond to the above-mentioned signal elements A to D in Segments 2 and 3.
More specifically, in Segment 2, an alternate pattern of points A and B is received.
FIG. 7B shows frequency components of the demodulated baseband signal at this time. As shown in FIG. 7B, the demodulated baseband signal in Segment 2 has line spectrums at .+-.f.sub.b /2=.+-.1,200 Hz as 1/2 the baud rate frequency and a DC frequency.
The pseudo random sequence in Segment 3 is generated using PN codes according to the above-mentioned polynomial. In the case of the V.29 modem, when the pseudo random sequence is "0", a point C is always transmitted; when the pseudo random sequence is "1", a point D is always transmitted. More specifically, Segment 3 starts with CDCDCDC . . . of the sequence, and continues during a 384-symbol interval.
In order to pull an automatic equalizer, the transmission-side modem transmits a pseudo random sequence signal, and the reception-side modem also generates the same signal. The automatic equalizer is initialized based on a difference signal between the pseudo random sequence signal input from the transmission-side modem to the reception-side modem and the signal generated by the reception-side modem.
In order to perform the above-mentioned operations, an alternate pattern signal must be reliably detected from input signals, and the start point of the random sequence signal must also be reliably detected.
The arrangement of a conventional training signal detection apparatus (pseudo random sequence signal detection apparatus) for a modem will be described below with reference to FIG. 8. FIG. 8 is a block diagram of a conventional training signal detection apparatus (pseudo random sequence signal detection apparatus) for a modem.
An analog reception signal S.sub.1 input to an analog input terminal 10 is converted into a digital signal S.sub.2 by an A/D converter 11, and the digital signal is supplied to an automatic gain controller (AGC) 12.
The circuit after the A/D converter 11 is often realized by a software program in a digital signal processor (DSP) in practice.
The AGC 12 converts the digital signal S.sub.2 into a signal S.sub.3 normalized to a proper power. A demodulator (DEM) 13 demodulates the signal S.sub.3 normalized to the proper power into a complex baseband signal S.sub.4. The demodulated complex baseband signal S.sub.4 is branched into two paths. One complex baseband signal S.sub.4 is filtered to a signal S.sub.5 by a filter 50.
The conventional apparatus uses, as the filter 50, a narrow-band filter (f.sub.b /2-BPF) having a frequency 1/2 the baud rate frequency f.sub.b as the central frequency of the passband. A power calculator 15' calculates a square of the absolute value of the signal S.sub.5 to obtain a signal S.sub.7.
The other demodulated complex baseband signal S.sub.4 is supplied to a power calculator 15. The calculator 15 calculates a square of the absolute value of the signal S.sub.4, and the signal S.sub.4 is multiplied with a positive constant .beta. by a constant multiplier 16 to obtain a signal S.sub.6. The two signals S.sub.6 and S.sub.7 are input to an adder/subtracter 17.
The adder/subtracter 17 calculates (signal S.sub.8)=(signal S.sub.7)-(signal S.sub.6) , and the signal S.sub.8 is smoothed by a low-pass filter (LPF) 18 to obtain a signal S.sub.9.
Since the power of the signal S.sub.3 is normalized by the AGC, the power of the demodulated signal S.sub.4 is normally constant independently of the signal segment. Therefore, the magnitude of the signal S.sub.5 is almost constant independently of the signal segment.
The signal S.sub.7 has a power of a frequency component 1/2 the baud rate frequency of the demodulated signal S.sub.4. Therefore, in the segment of an alternate pattern signal, the signal S.sub.7 assumes a considerably large value as compared to other segments.
Therefore, when .beta. is properly selected, the signal S.sub.8 assumes a positive value in the segment of the alternate pattern signal, and assumes a negative value in other segments.
Therefore, the alternate pattern signal can be detected by checking the sign of the signal S.sub.9 obtained by smoothing the signal S.sub.8 by a sign discriminator 19.
Note that the LPF 18 is arranged to eliminate the influence of inversion of the sign of the signal S.sub.8 due to, e.g., impulse noise.
In order to detect a subsequently sent pseudo random sequence, substantially the same arrangement as described above is used, except that the filter 50 comprises a band elimination filter (BEF) in place of the above-mentioned narrow-band filter. When this BEF is a filter having factors (1+Z.sup.-2)(1-Z.sup.-1) as a transfer function, and is operated at a sampling frequency 2f.sub.b, it can remove a frequency component 1/2 the baud rate frequency and a DC component. FIG. 9 shows frequency characteristics when the transfer function of the BEF is given by (1+Z.sup.-4)=(1+Z.sup.-2) (1-Z.sup.-1) (1+Z.sup.-1).
In the above-mentioned alternate pattern segment, the demodulated complex baseband signal S.sub.4 has line spectrums at the frequency 1/2 the baud rate frequency and a DC frequency. For this reason, when the filter 50 comprises the BEF, the output signal S.sub.5 becomes 0. However, when the alternate pattern segment transits to the pseudo random sequence segment, the signal S.sub.5 has a large value, as is well known. Therefore, in this case, the signal S.sub.7 also has a large value.
The power of the signal S.sub.3 is normalized by the AGC, and the magnitude of the signal S.sub.6 becomes almost constant independently of the signal segment, as described above. For this reason, when .beta. is properly selected, the sign of the output (signal S.sub.8)=(signal S.sub.7)-(signal S.sub.6) from the adder/subtracter 17 can be reliably changed from a negative value to a positive value at the beginning of a pseudo random sequence segment.
Therefore, when the sign of the signal S.sub.9 obtained by smoothing the signal S.sub.8 by the LPF 18 is checked by the sign discriminator 19, the start point of a pseudo random sequence segment can be detected.
However, in the above-mentioned arrangement, the narrow-band filter operation of the filter 50 in the alternate pattern segment detection processing delays transmission of a signal power. For this reason, the signal S.sub.7 follows a change in power of the demodulated signal S.sub.4 with a delay. For this reason, when the power of the signal S.sub.4 abruptly changes due to some cause, the sign of the signal S.sub.9 may be inverted.
For example, a case will be examined below wherein instantaneous disconnection occurs during data reception. When data is normally received, the sign of the signal S.sub.9 is negative. At this time, when a instantaneous disconnection occurs, the power of the signal S.sub.3 assumes a value very close to 0 since the AGC 12 cannot immediately increase the gain. During this interval, the signals S.sub.4 and S.sub.6 also assume values very close to 0.
However, since the signal S.sub.7 follows a change in power of the signal S.sub.4 with a delay, a situation of S.sub.7 &gt;S.sub.6 .apprxeq.0 occurs. At this time, the signal S.sub.9, which must be originally negative, becomes positive, and an alternate pattern signal is erroneously detected.
Thus, although data is being received, a controller of the modem forcibly initializes the various signal processors of the modem to a state before the beginning of training.
At this time, possibility that an equalizer adjustment signal is erroneously detected is also very high. The controller of the modem starts training of the automatic equalizer. However, in practice, since no equalizer adjustment signal is received, the equalizer is not trained but diverges.
In this manner, in a conventional alternate pattern segment detection method, when a signal power abruptly changes during data reception, an alternate pattern signal may be erroneously detected, and thereafter, the signal processors of the modem cannot be normally operated.
In control for detecting the start point of the pseudo random sequence segment, when a signal having a very low S/N ratio is received and demodulated, the signal S.sub.6 varies drastically, and an erroneous detection consequently occurs. In this case, the erroneous detection can be prevented by increasing the time constant of the LPF 18 . However, when the time constant of the LPF 18 is sufficiently increased, the sign of the signal S.sub.9 cannot be inverted to a positive value even when the signal S.sub.7 becomes large.
For this reason, when the time constant of the LPF 18 is increased too much, the start point of a pseudo random sequence cannot be detected.
As a result, the time constant of the LPF 18 cannot be increased too much, and an erroneous detection for a signal having a low S/N ratio cannot be effectively prevented.
When the start point of a pseudo random sequence cannot be normally detected, the automatic equalizer cannot be initialized.
When the equalization processing cannot be normally performed, received data completely loses its reliability, thus posing a very serious problem.